Quantum creep in Y1-xPrxBa2Cu3O7-delta crystals: Magnetic relaxation and transport

We report transport and magnetic relaxation measurements in the mixed state of strongly underdoped Y1-xPrxBa2CU3O7-delta crystals. A transition from t hermally activated flux creep to temperature-independent quantum flux creep is observed in both transport and magnetic relaxation at temperatures T le ss than or equal to 5 K. Flux transformer measurements indicate that the cr ossover to quantum creep is preceded by a coupling transition. Based on the se observations we argue that below the coupling transition the current is confined within a very narrow layer beneath the current contacts. [S0031-90 07(99)08855-9].